This is a clinical research study. Its purpose is to determine if electrophysiologic study (EPS) or holter monitoring (HM) is the most effective technique for selecting long term antiarrhythmic drug therapy in patients with sustained ventricular tachycardia, ventricular fibrillation, or an episode of aborted sudden death. This project will be carried out at the University of Utah and six cooperating Universities under a consortium arrangement. Patients are asked to sign consent if they fulfill the above clinical criteria. They then undergo 48 hours of ambulatory ECG tape recording. If they have at least an average of 30 premature ventricular contractions per minute they then go on EPS. If they have reproducibly inducible, sustained ventricular tachyarrhythmias they are randomized to undergo subsequent antiarrhythmic drug evaluation by either EPS or HM. They also undergo a treadmill test at this time. Each patient will receive up to five antiarrhythmic drugs (quinidine, procainamide, tocainide, imipramine, and sotalol). The order in which each patient receives these drugs will be randomized, and drugs which were previously ineffective will be excluded in specific cases. Each drug will be tested by the assigned technique until one is predicted to be effective. When a drug is found to be effective, the patient will undergo a second treadmill test and will be discharged for frequent follow-up holter monitoring. The primary end point in this study, upon which the superiority of one or the other testing method will be assessed, is arrhythmia recurrence, carefully defined in the proposal. Additional comparisons between the two testing methods will be made: 1) Cost, 2) drug efficacy rate, 3) length of hospital stay, and 4) accuracy of the prediction for each of the five drugs. The study is designed to evaluate concordance between EPS and HM predictions. Finally, the study will assess overall efficacy of each of the five drugs. Whether the hypothesis of this study is verified or not, the results of the study will alter clinical care of patients with ventricular tachyarrhythmias.